Colour picture tubes

ABSTRACT

In a colour picture tube having a colour selection electrode disposed to oppose the fluorescent screen of the tube and having an effective area provided with a plurality of perforations for transmitting electron beams, the effective area of the colour selection electrode is provided with a compensating mechanism for absorbing local thermal expansion of the effective area thereby preventing the deformation thereof.

Uite States Patent Yamazaki et a1.

COLOUR PICTURE TUBES Inventors: Eiichi Yamazaki, lchihara; Akio Yamaguchi, Mobara, both of Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Oct. 23, 1973 Appl. No.: 408,938

Foreign Application Priority Data Oct. 27, 1972 Japan 47-107203 u.s. Cl. 313/403, 313/402 Int. Cl. H0lj 29/80 Field of Search 313/853, 92 B, 402, 403

References Cited UNITED STATES PATENTS Green 313/85 S X Mar. 18, 1975 2,863,079 12/1958 Oestreicher 313/85 S 3,653,900 4/1972 Black 313/85 S X 3,772,555 11/1973 McKee et al..... 313/85 S Primary Examiner.lames B. Mullins Attorney, Agent, or FirmDike, Bronstein, Roberts,

' Cushman & Pfund [57] ABSTRACT In a colour picture tube having a colour selection electrode disposed to oppose the fluorescent screen of the tube and having an effective area provided with a plurality of perforations for transmitting electron beams, the effective area of the colour selection electrode is provided with a compensating mechanism for absorbing local thermal expansion of the effective area thereby preventing the deformation thereof.

7 Claims, 14 Drawing Figures sum 1 n; 4

F/ G PRIOR ART F/GZ COLOUR PICTURE TUBES BACKGROUND OF THE INVENTION This invention relates to a colour picture tube, more particularly to a colour selection electrode of a colour picture tube.

Among colour selection electrodes for use in colour picture tubes are included various types such as a shadow mask provided with a plurality of circular perforations for transmitting electron beams, a stripe mask provided with elongated openings or slits for transmitting electron beams or a grid type mask in which metal wires are secured to a frame. The colour selection electrode to which the invention is applicable has a construction wherein a plurality of perforations for transmitting electron beams in the form of regularly ar ranged circular, oblong or rectangular openings are provided for a plate, such as the shadow mask and stripe mask described above. For the sake of simplicity, in the following description, the invention will be described in connection with a colour selection electrode in the form of a shadow mask.

The so-called shadow mask type colour picture tube utilizing a shadow mask usually comprises an evacuated envelope including a panel, a funnel and neck, a fluorescent screen formed on the inner surface of the panel, a shadow mask, that is a colour selection electrode, spaced apart a predetermined distance from the fluorescent screen, and an electron gun assembly contained in the neck. The electron beams emitted by the electron gun assembly are deflected by a deflection coil surrounding the envelope and selected by the shadow mask for different colours. The electron beams transmitting through the shadow mask are caused to impinge upon predetermined phosphor dots on the fluorescent screen, thus causing the dots to fluoresce.

In the shadow mask type colour picture tube described above, since the colour selection is made clue to parallax the percentage of transmission of the shadow mask is at most 15 to 25 percent, and theoretically it is impossible to increase the percentage of transmission beyond 33.3 percent. Consequently, a substantial number of electrons accelerated to high speed collide upon the shadow mask to give their energy to the shadow mask thus raising the temperature thereof. It has been recognized that the expansion of the shadow mask caused by such temperature rise causes colour mismatch. To compensate for such colour mismatch bimetals-have been used to mount the colour selection electrode upon a supporting frame so as to move the shadow mask toward the fluorescent screen as the shadowmask undergoes thermal expansion.

The purpose of the bimetals, however, is to provide a compensation when the assembly of the shadow mask and the supporting frame undergoes thermal expansion as an integral unit so that it is impossible to perfectly compensate for the colour mismatch by such measure.

As will be described later with reference to the accompanying drawings, where the entire surface of the picture has uniform brightness on where the density of current flowing into the fluorescent screen is substantially constant, the thermal expansion of the shadow mask is substantially constant over its entire surface so that it is possible to perfectly compensate for the colour mismatch by using bimetals for mounting the shadow mask on the supporting frame. However, where the brightness of the picture is not uniform over the entire surface thereof, or where the density of current flowing into the fluorescent screen is not uniform over the entire surface and where the image is extremely bright at some portion of the fluorescent screen, the density of current flowing into the shadow mask is different from point to point with the result that a portion of the shadow mask expands locally. Consequently, the surface of the shadow mask which is shaped to a predetermined curved surface and provided with a plurality of perforations for transmitting electron beams is deformed to depart from said predetermined curved surface. This forms a colour mismatch on the fluorescent screen corresponding to the deformation thereby greatly degrading the quality of the reproduced picture. The bimetals described above is useless to compensate for this phenomenon. The deformation caused by the local expansion of mask face or the effective area of the shadow mask is temporal in one case whereas it is permanent in the other case. In the latter case, the colour mismatch occurs always irrespective to the temperature condition of the shadow mask which is detrimental to the colour picture tube.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved colour picture tube provided with a novel colour selection electrode capable of preventing deformation of the effective area of the colour selection electrode caused by local thermal expansion.

A further object of this invention is to provide a novel compensating mechanism which is easy to manufacture and capable of preventing the deformation of the effective area of the colour selection electrode caused by local thermal expansion.

According to this invention these and further objects can be accomplished by providing a colour picture tube of the class comprising an evacuated envelope, a fluo rescent screen disposed in the envelope, a colour selection electrode disposed to oppose the fluorescent screen and including an effective area provided with a plurality of perforations for transmitting electron beams, and an electron gun assembly, characterized-by a compensating mechanism provided for the effective area of the colour selection electrode for absorbing local thermal expansion of the effective area thereby preventing deformation thereof.

The compensating mechanism can be most conveniently formed by corrugating the effective area or by providing bottomed slits for the' effective area, the bottomed slits extending towards each other from the opposite surfaces of the effective area.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a side view, partly broken away, of one example of a prior art colour picture tube illustrated for the purpose of explaining the deformation of the effective area of the colour selection electrode;

FIG. 2 is an enlarged sectional view of a portion of the tube shown in FIG. 1;

FIGS. 3A and 3B are diagrams to compare the deformation of the effective area of the prior art construction and of a shadow mask embodying the invention caused by local thermal expansion;

FIG. 4 shows a perspective view of a portion of one example of the compensating mechanism utilized in the colour picture tube of this invention;

FIGS. 5A and 5B are enlarged views of a portion of the compensating mechanism shown in FIG. 4;

FIG. 6A shows a plan view of a modified example of the compensating mechanism utilized in the colour picture tube of this invention;

FIG. 6B shows a cross-sectional view taken along a line VI-VI shownin FIG. 6A;

FIG. 6C is a diagram showing the relationship between the compensating mechanism shown in FIG. 6A and the perforations thereof for transmitting electron beams.

FIG. 7 is a partial cross-sectional view of the shadow mask provided with the compensating mechanism shown in FIG-6A which is helpful to explain the deformation of the effective area of the shadow mask caused by thermal expansion;

FIG. 8A is a plan view showing another example of the compensating mechanism utilized in the colour picture tube of this invention;

FIG. 8B is a sectional view taken along a line VIII- VIlI shown in FIG. 8A and FIG. 9 is a partial plan view showing another arrangement of the perforations for transmitting electron beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A priorart colour picture tube shown in FIG. 1 comprises an evacuated envelope including a panel 2, a funnel 3, and a neck 4. On the inner surface of the panel 2 is formed a fluorescent screen 5 and a colour selection electrode 6 shown as a shadow mask is mounted on a supporting frame 7 and is disposed close to the fluorescent screen 5. The supporting frame 7 and the shadow mask 6 supported'thereby are secured to the inner wall of the funnel 3 by means of a plurality of supporting members 8 in the form of bimetals or members including bimetals at one portion thereof, one end of each supporting member 8 being secured to the supporting frame 7 and the other end being provided with an opening (not shown) for receiving a pin 9 secured to the inner side of the funnel. As usual, an electron gun assembly 10 is contained in the neck 4 to emit electron beams 11 which transmit through perforations 6a provided for the shadow mask 6 to impinge upon the desired phosphor dots (not shown) formed on the fluorescent screen 5, thus causing the phosphor dots to luminesce. A deflection coil 12 is mounted on the outside of the envelope 1.

As best shown in FIG. 2, a plurality of perforations 6a for transmitting the electron beams are provided for the effective area 13 of the shadow mask 6, it being understood that such perforations are not provided for the periphery 15 of the shadow mask between its effective area 13 and a skirt 15. Dotted lines 16 show the local thermal expansion of the effective area.

In operation, the electron beams 11 emitted from the electron gun assembly 10 are deflected by deflection coil 12 to transmit through selected one of the perforations 6a of the shadow mask to impinge upon desired phosphor dots on the fluorescent screen. Where the brightness of the picture is substantially uniform over the entire area, or the density of current flowing into the fluorescent screen is substantially constant, the shadow mask 6 will undergo substantially uniform thermal expansion. Accordingly, it is possible to provide substantially perfect compensation for colour mismatch by the action of the bimetallic supporting members.

However, where the brightness of the picture is not substantially uniform over the entire area or where the density of the current flowing into the fluorescent screen 5 is not uniform over the entire area, with the result that the brightness of a portion of the image reproduced on the fluorescent screen 5 is extremely bright, and the density of the current flowing ino the shadow mask differs from point to point.

Assume now that the current density is high at the portion 16 of the shadow mask, this portion expands locally thus deforming the effective area 13 which have been shaped into a predetermined curved surface. This results in a colour mismatch on the portion of the fluorescent screen corresponding to the deformed portion, thus greatly degrading the quality of the picture. As described above, the compensation provided by the bimetallic supporting members is not effective entirely for this phenomenon.

The phenomenon will be considered in more detail with reference to FIG. 2. An electron beam 11a1 transmitted through a perforation 6a1 of shadow mask 6 under the normal condition impinges upon a phosphor dot 17 on the fluorescent screen, but when a portion of the effective area 13 undergoes local expansion thus deforming the effective area to a position shown by dot and dash lines the electron beam 11a2 transmitting through the perforation 6al whose position has been shifted due to said deformation will impinge upon a phosphor dot 18 which is different from phosphor dot 17 thus causing the dot 18 to luminesce. This of course results in undesirable colour mismatch. Since the bimetallic members operate solely dependent upon their own temperature independent of the local temperature rise of the effective area, they can not follow the local temperature rise described above. In other words, the colour mismatch caused by the local expansion can not be prevented by conventional bimetallic members.

With reference now to FIGS. 3A dnd 38 the unidimensional deformation due to local thermal expansion of a curved surface with its opposite ends held stationary will be discussed. It is supposed now that a portion 1 of a uniformely curved surface maintained at a predetermined temperature is deformed by h to a position l due to local thermal expansion as shown in FIG. 3A. When a corrugated curved surface shown in FIG. 3B made of the same material and maintained at the same temperature as the curved surface shown in FIG. 3A undergoes local thermal expansion, the curved surface will expand until the total sum of micro arcuate lengths of respective curved surfaces becomes equal to the length of an are 1 shown in FIG. 3A, the extent of deformation h of the effective area in this case being much smaller than h in the previous case. Based on this consideration, the colour selection electrode of this invention is provided with the corrugation or an equivalent compensating mechanism at the effective area thereof.

FIG. 4 shows one example of the shadow mask provided with the compensating mechanism of this invention, in which shadings are applied on the assumption that the light is projected from the upper right. The effective area 23 of the shadow mask 26 shown in FIG. 4 is shaped to have a wavy configuration which when viewed normal to the mask surface appears as a corrugated surface of contiguous concave-convex areas to provide a compensating mechanism 19 free to expand and contract two dimensionally. More particularly, in the compensating mechanism 19, all lines la interconnecting points a are at the same level and all lines 1b interconnecting points b are also at the same level, the lines 1a being positioned at the tops whereas lines 1b at the valleys. Accordingly, points b are positioned at lower levels than points a. The pitch of the corrugations is determined by taking into such factors as the size of the effective area 23, the radius of curvature thereof, and the maximum operating temperature.

FIGS. 5A and 5B show a plan view and a side view of the compensating mechanism shown in FIG. 4. In FIGS. 5A and SB, g represents the distance between points a, and a h the height of the top, that is the vertical height between points a and b and f the distance between points b and b Where the distances 3 andfare varied to g and f respectively, h will bevaried to h, meaning that it is possible to vary distances g and f without creating any appreciable internal stress. The same relation holds true when g and f increase. In other words, even when there is a tendency of increasing g and f due to thermal expansion, if the periphery of the effective area is held stationary, only the height varies. Accordingly, in the shadow mask provided with the compensating mechanism shown in FIG. 4, since the variation of g and f due to thermal expansion occurs in each element, such variation and hence the variation of h is small, thus preventing any appreciable deformation of the effective area. Actually, the inclination angle of oblique lines lab interconnecting points a and b is very small and the length of the line lab is of the order of 0.6 mm. The local deformation, or the distance it, shown in FIG. 3A, due to local heating of a shadow mask not provided with any compensating mechanism is about 0.2 to 0.3 mm, whereas in the shadow mask provided with the compensating mechanism it is possible to reduce the height h by several orders of magnitude. The perforations for transmitting electron beams may be provided at points a or in the inclined surfaces bounded by oblique lines lab interconnecting points a and b. However, it is advantageous to locate the perforations at points a in order to facilitate bending of the effective area along the top and bottom edges as well as the oblique lines lab. The illustrated compensating mechanism 19 can be readily formed by a press work, for example.

FIGS. 6A, 6B and 6C show an effective area of the shadow mask provided with a modified compensating mechanism. The modified compensating mechanism 39 comprises bottomed slits 34 at portions of the shadow mask 36 corresponding to the top and bottom edges and the oblique lines shown in FIG. 4, the slits extending inwardly from the opposite surfaces of the effective area 33. As can be noted from FIG. 7, this modified compensating mechanism has substantially the same function as that shown in FIG. 4 although its construction is simpler.

With this modified construction since the slits 34 act as the absorber of expansion to permit free expansion and contraction it is possible to prevent any appreciable deformation of the effective area 33. The width of the perforations'35 for transmitting the electron beams are formed at the intersecting points of respective slits 34. However, it is to be understood that it is not always necessary to form the perforations at such intersections and that such perforations may be at intermediate points of the slits or in intermediate portions 38 bounded by the slits 34. The area of such intermediate portions may be varied to form one, two or more than two perforations in each portion or may be small not to permit provision of a single perforation. The area is determined in accordance with the size of the shadow mask. Usually it is advantageous to form the perforations 35 at the intersections of the slits for the reason described above. It is advantageous to form the slits 34 concurrently with the perforations for transmitting the electron beams. Generally, the perforations 35 are formed by photoetching technique.

FIGS. 8A and 8B illustrate still further modification of the compensating mechanism 49 in which the perforations 45 for transmitting the electron beams are directed in different directions whereas in the example shown in FIG. 6, the perforations 35 are directed in the same direction with their smaller openings opened upwardly. Furthermore, the bottomed slits 44 are provided between adjacent perforations 45.

FIG. 9 shows another arrangement of the perforations for transmitting the electron beams wherein shadow mask 5b is a modification of the shadow mask shown in FIG. 4.As shown, a plurality of perforations 55 for transmitting the electron beams are formed in each inclined surface bounded by lines la, lb and lab. The effective area 53 of the shadow mask is corrugated to permit two dimensional expansion and contraction. With this construction, the length of lines 1a, 1b and lab is preferably to be 0.5 to 50 mm. However, if the length were too long, the compensating effect for local expansion would be decreased. Such modified compensating mechanism can be formed simultaneously. with the shadow mask as by press work.

Although in the foregoing description the invention was applied to shadow masks having circular perforations for transmitting the electron beams, it should be understood that the colour selection electrode is never limited to a shadow mask and that such perforations may be elongated circles or slits, that is a stripe mask. Further, in the embodiments shown in FIGS. 4 and 9, lines la, lb were shown as continuous folds, whereas as slits in the embodiments shown in FIGS. 6A, 6B and 8A, 8B, they may be of any suitable form so long as equivalent effect can be obtained. Thus for example, bottomed perforations having suitable spacings may be formed along said lines or slits. The invention can also be applied to a colour selection electrode not provided with a supporting frame.

As has been described in detail the invention pro.- vides a colour picture tube provided with an improved colour selection electrode having a compensating mechanism provided for the surface or effective area of the colour selection electrode including a plurality of perforations for transmitting the electron beams, the compensating mechanism functioning to absorb local thermal expansion of the effective area so as to efficiently prevent the effective area from deforming. Ac-

cordingly, it is possible to prevent colour mismatch of the reproduced colour picture caused by the deformation of the effective area of the colour selection electrode, thereby improving the colour purity of the reproduced image. Since the compensating mechanism can be formed concurrently with the perforations for transmitting the electron beams, or concurrently with the shaping operation of the colour selection electrode, it is possible to manufacture the compensating mechanism at a low cost.

What is claimed is:

1. In a colour picture tube of the class comprising an evacuated envelope, a fluorescent screen disposed in said envelope, a colour selection electrode disposed to oppose said fluorescent screen and including an effective area provided with a plurality of perforations for transmitting electron beams, and an electron gun assembly, the improvement which comprises a compensating mechanism having a corrugated surface of contiguous concave-convex areas substantially constituting said effective area of said colour selection electrode for absorbing local thermal expansion of said effective area thereby preventing deformation of said effective area.

2. The colour picutre tube according to claim 1 wherein said compensating mechanism comprises corrugations formed on said effective area, said corrugations having a plurality of spaced apart top edges on the same curved surface and a plurality of bottom edges parallel to said topedges and positioned intermediate adjacent top edges;

3. The colour picture tube according to claim 1 wherein said compensating mechanism comprises corrugations formed by shaping said effective area and bottomed slits provided at respective folds of said corrugations, said bottomed slits extending toward each other from the opposite surfaces of said effective area.

4. The colour picture tube according to claim 2 wherein said perforations for transmitting electron beams are provided on said top and bottom edges.

5. The colour picture tube according to claim 3 wherein said perforations for transmitting the electron beams are provided along said slits.

6. The colour picture tube according to claim 1 wherein each of said perforations having a larger opening on one side and a smaller opening on the other side, and said larger openings and smaller openings are disposed alternately on both sides of said effective area.

7. The colour picture tube according to claim 1 wherein bottomed slits are formed in the walls between adjacent perforations and the bottomed slits in adjacent walls extend in the opposite directions from the opposite surfaces of said effective area. 

1. In a colour picture tube of the class comprising an evacuated envelope, a fluorescent screen disposed in said envelope, a colour selection electrode disposed to oppose said fluorescent screen and including an effective area provided with a plurality of perforations for transmitting electron beams, and an electron gun assembly, the improvement which comprises a compensating mechanism having a corrugated surface of contiguous concaveconvex areas substantially constituting said effective area of said colour selection electrode for absorbing local thermal expansion of said effective area thereby preventing deformation of said effective area.
 2. The colour picutre tube according to claim 1 wherein said compensating mechanism comprises corrugations formed on said effective area, said corrugations having a plurality of spaced apart top edges on the same curved surface and a plurality of bottom edges parallel to said top edges and positioned intermediate adjacent top edges.
 3. The colour picture tube according to claim 1 wherein said compensating mechanism comprises corrugations formed by shaping said effective area and bottomed slits provided at respective folds of said corrugations, said bottomed slits extending toward each other from the opposite surfaces of said effective area.
 4. The colour picture tube according to claim 2 wherein said perforations for transmitting electron beams are provided on said top and bottom edges.
 5. The colour picture tube according to claim 3 wherein said perforations for transmitting the electron beams are provided along said slits.
 6. The colour picture tube according to claim 1 wherein each of said perforations having a larger opening on one side and a smaller opening on the other side, and said larger openings and smaller Openings are disposed alternately on both sides of said effective area.
 7. The colour picture tube according to claim 1 wherein bottomed slits are formed in the walls between adjacent perforations and the bottomed slits in adjacent walls extend in the opposite directions from the opposite surfaces of said effective area. 